Prior art techniques provide a variety of forms of timers or delay circuits in which an output is produced a selected time after an input stimulus. Typically, these prior art devices employ one of two technologies. In one device, the time delay is measured with the aid of an electro-mechanical motor driven timer. The other technology which has been employed to produce delay in devices is analog circuitry in which usually a capacitor is charged up to some target voltage, and the charging process takes a controllable amount of time which corresponds to the delay.
A subset of the applications for timers requires not only practical devices, but devices which exhibit fail-safe or vital characteristics in that they will not, except under very unusual circumstances, provide a time delay less than the selected time delay. Almost universally, the motor driven timer technology is employed in this subset of the field. A vital timer employing analog techniques is disclosed and claimed in the application of Auer et al, filed Jan. 30, 1978, (GR-424), Ser. No. 874,007, entitled "Fail-Safe Time Delay Circuit."
However, digital techniques and in particular, microprocessor devices, have characteristics which make them attractive as the timing element in a delay circuit. For one thing, the microprocessor is capable of accurately timing long intervals. Timing loops can be constructed with selected instruction steps and arranged to take a definite number of clock counts. Although the clock counts are at relatively high frequencies, since the processor can keep track of large numbers easily, accurate timing of even long intervals is possible. In contrast, analog techniques, when timing relatively long intervals, require threshold detection at levels which have inherently large setability tolerance. For example, an analog device with relatively good repeatability of .+-.1% will, in timing an 8 minute interval, provide time delays that vary over almost 10 seconds. On the other hand, digital techniques, if employed, would be expected to have timing repeatability which is not a function of the period being timed. For example, a reasonable goal appears to be .+-.1/2 second and thus, in timing out large intervals, would exhibit much better repeatability than the analog device.
However, especially when a vital or fail-safe characteristic is required, checking techniques must be employed to insure that the microprocessor has actually executed the program steps required as a prerequisite to insuring that the desired delay is actually exired before outputting a signal. In addition, since the delay will be a function of a certain number of clock counts, it is also important to be able to check the accuracy of the microprocessor clock, i.e., to guard against clock drift reducing the actual delay below the desired delay.
It is therefore one object of the present invention to provide a digital delay circuit which exhibits vital or fail-safe characteristics. It is another object of the invention to provide such a circuit which includes, as a major component thereof, a conventional microprocessor. It is yet another object of the present invention to provide a vital or fail-safe time delay circuit including a microprocessor as a major component thereof in which the instruction steps which direct the microprocessor to time out the desired delay, include several instruction steps which act to check the proper operation of the microprocessor. It is a further object of the present invention to provide a circuit of the foregoing type which further includes apparatus specifically arranged to monitor the proper operation of the microprocessor clock which apparatus is capable of suppressing an output signal in the event that the microprocessor clock drifts beyond a predetermined range from its nominal frequency. It is another object of the present invention to provide apparatus for checking the proper operation of a microprocessor clock to insure that the microprocessor output is not employed by any utilization circuits unless the microprocessor clock is operating within a desired tolerance of its nominal frequency.